Better Roads

November 2013

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HighwayContractor
By Jack Roberts, Truck Editor
Allison's automatic transmission features
fluid drive and a torque converter. Adherents
argue these provide 100-percent power
and double rim pull, making them ideal for
vocational applications.
Automatic or Manual?
More choices — and it's not just what you 'feel' like
R
evolutions can be subtle things. Unless there's an
army or angry mob storming down Main Street,
it's not always easy to tell when you're in one.
That's what makes trucks today so interesting.
The advent of automatic and automated transmissions in
heavy-duty commercial vehicles is changing things. In many
ways, the explosion in popularity of these units during the
past decade is an offshoot of the rapid evolution being seen
in onboard vehicle computers.
Automatic transmissions are nothing new. They've been
around in automotive applications in one form or the other
since the 1940s. But adapting them for use in heavy-duty
trucks proved to be difﬁcult for a variety of reasons – not the
least of which was the inability of mechanical engines and
transmissions to "talk" to one another and ensure optimal
vehicle performance in the bewildering array of applications,
road conditions and cargo/payload conﬁgurations.
Early attempts to develop and introduce them met with
less-than-acceptable results. There were durability issues. But
most complaints tended to focus on performance issues.
Before the arrival of today's powerful and compact electroniccontrol modules, early transmissions had a hard time ﬁguring
out what drivers wanted. Complaints concerning frequent,
uneven shifts while accelerating were common, as were prob26 November 2013 Better Roads
lems with "searching," when transmissions would struggle in
hilly terrain to ﬁnd – and stick with – an optimal gear. Other
problems along those lines emerged at low speeds.
Dry Clutches and Torque Convertors
Although the term "automatic" is being used to refer to all
non-manual gearboxes, it's important to note there are actually two types of automatic transmissions available for heavyduty trucks: automated manual and automatic transmissions.
Both types are two-pedal designs, and most drivers would be
hard-pressed to tell much of a difference between them.
The basic difference is automated manuals are manual
gearboxes with all the clutch actuation and gearshifts
handled by electronically controlled systems. True automatic
transmissions feature planetary gearing with disc packs for
clutches and with torque converters.
"In this context, an automatic for a Class 8 truck is usually
fully automatic, like a typical car transmission, with planetary gearing with several multi-disc packs for clutches," says
Ed Saxman, product marketing manager with Volvo Trucks
North America. "These transmissions have a torque converter
to enable powershifts of the planetary epicyclic gearing units
that provide the various gear ratios. An automated manual
transmission uses the gearbox of a manual transmission and